The invention relates to a method of Gas-shielded Metal Arc joining with a consumable electrode having an alternating polarity (GMA-AC).
Gas-shielded metal arc joining comprises the joining of metals by welding, brazing or combinations of these methods using a shielding gas. For years, gas-shielded metal arc welding (GMA welding or GMAW) has been the predominantly used method of joining metallic materials and is defined and described in International Standards Organization Publication ISO 857. In GMA welding, filler material is fed in an arc which burns between a consumable electrode and one or more workpieces. In the process, a connection is created with the base material, which also undergoes melting. Depending on the type of shielding gas used, the process can be characterized as welding with an inert gas or welding with an active gas. The shielding gas selection depends on the materials to be welded.
For several years, GMA brazing has also been practiced as a variant of the GMA welding process. Here, a wire electrode is used which has a low melting point. The object of the process is to make a brazed joint with no melting of the base material, if possible, or with only slight melting of the base material. More recently, a combination of both processes has also been attempted, such as for joining aluminum materials to coated steel plates. In this case, the process more closely resembles a brazing process on one side of the material, while it more closely resembles a welding process on the other side. All of these processes are carried out using direct current (DC) or pulsed direct current. An example of this type of process is described in European Patent Document EP 1 129 808 A1. In most applications, the consumable electrode is positively polarized, and the base material is negatively polarized. This type of polarization ensures a sufficient pinching of the liquid-pasty molten metal of the electrode by way of the so-called “pinch force”. In processes with pulsed current, this is a prerequisite for a regular and reproducible material transfer. On the base material side, the negative polarization causes an electron emission which contributes to the heating and melting of the metal, either with respect to the surface or by way of drifting points. In a few exceptional cases, a negatively polarized, consumable electrode is used. These are special applications, such as build-up welding or welding with certain filler wires.
For several years, it has also been attempted to let the filler material be consumed using a current that provides an alternating polarity; see European Patent Document 1 103 329 A2 or International Institute of Welding Document No. XII-1720-02 of May 2002.
The following are the advantages of this process: Better preheating of the wire electrode during the negatively polarized phase and, as a result, a higher capacity for consuming the wire electrode; better ability to bridge gaps for thin components; and the ability to use GMA welding for sheets that were too thin using previous techniques. Because of these advantages, the new process is receiving much attention particularly in the automobile industry. Currently, two different equipment types are already on the market; additional suppliers are expected soon. Research organizations and institutes expect a far-reaching potential for the GMA-AC process. The further developments of the following process/material combinations are of particular interest:                AC-GMA welding with an inert shielding gas of aluminum alloys,        AC-GMA welding of coated and uncoated steel,        AC-GMA welding with an active shielding gas of thin steel plates,        AC-GMA joining (“braze-welding”) of aluminum with coated and uncoated steel plates,        AC-GMA welding of heat-sensitive materials,        AC-GMA build-up welding.        
As a result of the use of the AC technique, an improved consuming capacity without increased heating of the base material is expected. Better gap bridgeability is also expected, as well as the possibility of using wire electrodes having a larger diameter, which are more stable when transported.
In the previous publications, only the gases known from conventional DC processes were used in the application of the AC technique. It does not appear that the influence of shielding gases in AC-GMA welding and brazing has been investigated.
It is known that joining using a relatively cold arc, and thus a low introduction of heat into the base material, also has disadvantages. The wetting behavior is poor, and the joining seam can have a convex profile. The very narrow seam may, in turn, have a negative effect on the gap bridgeability. In the case of dynamically stressed parts, a convex seam also results in a geometric notch with correspondingly negative effects on the service life of the component. In the case of materials which tend to form pores, such as aluminum alloys, the frequency of pores increases because the gases are frozen into the fast-cooling molten metal. If insufficient wetting/melting is achieved at the surface of the components, rejects are obtained as a result of the lack of fusion at various locations, such as in the root layer, in the side wall, or between two layers. Based on the objective of increasing the productivity in today's modern manufacturing, joining speeds are constantly increasing. However, increasing the welding travel speed or brazing travel speed leads to an intensification of the above-mentioned disadvantages. Thus, the main advantage of the AC process—the low introduction of heat into the base material—is a disadvantage with respect to possible speeds, particularly in the case of fully mechanical robot joining techniques.